1. Field of the Invention
The present invention relates generally to a sheet stacking apparatus in which sheets are sequentially delivered to and stacked on a sheet stacking tray, and more particularly to a sheet stacking apparatus capable of guiding the sheets to a alignment stopper surface on the sheet stacking tray and pressing the sheets against on the sheet stacking tray by substantially constant pressing force irrespective of a sheet stacking height, also to a sheet processing apparatus of which an apparatus main body includes the present sheet stacking apparatus, and further to an image forming apparatus.
2. Related Background Art
An image forming apparatus that forms an image on a sheet, a binding device that binds a sheet bundle and a sheet processing apparatus that processes the sheets such as a punching apparatus that punches the sheet bundle, have hitherto included a sheet stacking apparatus sequentially stacked with the sheets. In this case, the majority of these apparatuses have a sheet tacking apparatus in which the sheets are stacked on the sheet stacking tray in a way that reduces floating of the sheet in consideration of curling of the sheet. Note that the image forming apparatus is exemplified such as a copying machine a laser beam printer, a facsimile and a multifunction apparatus having functions of these apparatuses.
A sheet stacking apparatus 1302 shown in FIG. 45 is provided in a binding device 1300 for binding the sheets formed with images. This sheet stacking apparatus 1302 is constructed so that a sheet bundle is formed by sequentially stacking the sheets on an intermediate process tray 1330 defined as a sheet stacking tray provided within a binding device 1300, a stapler 1301 moving along an edge of the sheet bundle staples the sheet bundle at a plurality of points, and thereafter the stapled sheet bundle is delivered to a stack tray 1622 from the intermediate process tray 1330.
An operation of binding the unillustrated sheet bundle at two points, which is stacked on the intermediate process tray 1330, will be explained with reference to FIG. 46.
The sheets discharged out of the apparatus main body of the image forming apparatus are delivered sheet by sheet to the intermediate process tray 1330, and upstream-side ends (trailing ends) of the sheets in a sheet delivery direction are made to abut on an abutment support surface 1331a of a trailing end stopper 1331 by a pull-in paddle 1360 rotating in an arrow-headed direction and a knurled belt 1608 of a delivery roller 1320a configuring a first delivery roller pair 1320, thereby aligning the trailing ends of the sheets. Further, both sides of the sheets are widthwise aligned by first and second alignment members 1340, 1341 getting close to the sheets from the both sides of the sheets.
With a repetition of these operations, the plurality of sheets stacked on the intermediate process tray 1330 becomes a sheet bundle of which the trailing ends and the side ends are aligned. Thereafter, the on-standby stapler 1301 moves to first and second stapling positions along the trailing ends of the sheet bundle, and staples the sheet bundle in the respective stapling positions.
Thus, the sheet stacking apparatus 1302 including the intermediate process tray 1330 is configured in the way that alignment of the trailing end portions of the sheet involves pulling back the sheets toward the trailing end stopper 1331 by a pullback mechanism composed of the pull-in paddle 1360, the knurled belt 1608, etc and making the sheets abut on the trailing end stopper 1331. Normally, the sheet formed with a toner image might, however, change in its water content amount when the toner image is heated, pressurized and thus fixed by a fixing unit within the apparatus main body of the image forming apparatus, or depending on a using environment. Therefore, a buckling phenomenon occurs on the sheet. Namely, the sheet might be warped (curled).
The thus-curled sheet and a sheet that is small in thickness and low in rigidity are, when abutting on the trailing end stopper 1331 on the intermediate process tray 1330, buckled and disturb the alignment of the sheet bundle. Such being the case, the following configurations of the trailing end stopper are taken as measures against a failure of alignment of the sheets on the intermediate process tray 1330.
A trailing end stopper 1332 shown in FIG. 47 includes an abutting surface 1332a provided with a plurality of grooves 1332b and a plurality of protruded strip members 1332c, which prevent the sheet from being pushed up along the abutting surface 1332a. 
A trailing end stopper 1333 shown in FIG. 48 includes a downwardly-inclined elastic film 1334 provided at an upper portion thereof, thereby invariably pressing the sheet trailing end portion against on the intermediate process tray 1330 (see FIG. 46).
A trailing end stopper 1335 shown in FIG. 49 includes a movable pressing member 1336 that presses the sheet in the vicinity of the trailing end stopper 1335. The pressing member 1336, after the sheets have abutted on the trailing end stopper 1335, presses the sheet bundle against on the intermediate process tray 1330, thereby flattening the sheets in a way that presses down the curled sheets (refer to Japanese Patent Application Laid-Open No. H11-130338).
The trailing end stopper 1332 illustrated in FIG. 47 takes a low-cost/space-saving structure with a small number of components, however, is unable to positively flatten the sheet by pressing the curl, since the curled sheet is held in an as-curled state by the grooves 1332b and the protruded strip members 1332c. 
Further, such poor alignment property arose that a deviation occurs in the aligning position of the trailing end of the sheet due to recessed and protruded portions of the plurality of grooves 1332b and the plurality-of protruded strip members 1332c. 
Further, when the widthwise alignment of the sheet is done by the first and second alignment members 1340, 1341 (see FIG. 46), it follows that the sheet moves in the widthwise direction in a state where the trailing end of each sheet enters the recessed portion of the groove 1332b and the recessed portion between the protruded strip members 1332c. Consequently, frictional resistance occurs between the sheet railing end and the recessed portion and retards the widthwise movement of the sheet, and the sheet can not be aligned widthwise exactly. Moreover, when the number of stacked sheets increases, an excessive load is applied to a motor for moving the first and second alignment members 1340, 1341 due to the frictional resistance, which is a cause for deteriorating the widthwise alignment property.
The trailing end stopper 1333 shown in FIG. 48 has a simple structure including the elastic film 1334 and is capable of enhancing the alignment property of the trailing end of the sheet by pressing down the curled sheet. The elastic film 1334 has elasticity, and hence, when the number of the stacked sheets increases, the pressing force rises when the sheet enters. Conversely, if set so that the pressing force of the elastic film 1334 becomes proper when the number of the stacked sheets increases, the elastic film 1334 lacks the pressing force when the number of the stacked sheets decreases. Therefore, a relationship between the pressing force when the sheet enters and the pullback force of the knurled belt 1608 gets unstable.
For example, as shown in FIG. 50A, if the pressing force by the elastic film 1334 becomes larger than the pullback force of the knurled belt 1608 when the number of the stacked sheets rises, an upper sheet Pa is unable to reach the trailing end stopper 1333, and the alignment property of the sheet trailing end can not be enhanced. Further, as illustrated in FIG. 50B, when the number of the stacked sheets decreases, the pressing force by the elastic film 1334 becomes smaller than the pullback force of the knurled belt 1608, and almost no pressing force acts on the sheet. Hence, the sheet Pa might be pushed up and buckled along the abutting support surface 1333a. It is considered for preventing the buckling to increase the elastic force of the elastic film 1334. If the elastic force is increased, however, it follows that the phenomenon illustrated in FIG. 50A occurs also when the number of the stacked sheets decreases. Therefore, such a problem arises that a pullback optimal area depicted in a hatching portion in FIG. 51 is narrowed. Moreover, the buckling property and the pull back resistance change on a sheet-by-sheet basis depending on surface resistance based on a sheet material and on a difference in thickness, and hence it is highly difficult to have configurations flexible to various types of sheets.
Further, in the trailing end stopper 1335 shown in FIG. 49, the pressing member 1336 stands by in positions designated by Al, A2, A3 corresponding to a sheet stacking height in the vicinity of the trailing end stopper 1335 and descends, thus pressing the sheet P. Therefore, the pressing member 1336 stands by in any one of these positions, and, when a gap between the uppermost sheet and an undersurface 1336a of the pressing member 1336 gets narrowed due to the increase in the number of the stacked sheets, the largely curled sheet is hard to enter this gap and might be jammed therein.
Moreover, the pressing member 1336 is constructed to move up and down perpendicularly to the sheet in the vicinity of the trailing end stopper 1335. Hence, in a view of the trailing end stopper as viewed from the front surface thereof in FIG. 49, it is required that a position of the stapler 1301 (see FIG. 46) and a position of the pressing member 1336 be deviated in the right-and-left directions in FIG. 49. Accordingly, in order for the pressing member 1336 to press the most effective and closest portion to the trailing end stopper, the stapler 1301 must be retreated backward (on the right side in FIG. 49) not to interfere with the pressing member 1336 when the pressing member 1336 performs pressing or must be provided outside the stapling positions of the stapler in the direction perpendicular to the sheet surface in FIG. 49.
Moreover, the pressing member 1336 shown in FIG. 49, if scheming to press the entire sheet widthwise area, is upsized in the retreat configuration described above and is therefore often disposed at a central portion serving as a non-stapling area, wherein the pressing member 1336 is unable to press the both end portions of the sheet which correspond to the stapling positions, and there is a case of causing a decline of the alignment property about the both side ends of the trailing end of the sheet.
Further, when in a two-point stapling mode, the stapler passes through the central portion of the trailing end of the sheet and moves to the stapling positions on a deep side. Hence, if the pressing member 1336 is disposed at the central portion, after temporarily retreating backward (on the right side in FIG. 49), the stapler moves to a position that is not overlapped as viewed from the front surface and further moves on the deep side. Moreover, the operation is also the same when moving to an anterior side from the deep side. Therefore, the stapler moving mechanism is required to enable the stapler to move in biaxial (X-axis, Y-axis) directions, and consequently the structure gets complicated. Moreover, the stapler moves in the biaxial directions, so that staple processing time elongates. Still further, standby time of the image forming apparatus increases corresponding to this elongated processing time, and productivity for forming the images decreases. It should be noted that the sheet stacking apparatus illustrated in FIG. 49 configures, together with the stapler 1301, a sheet processing apparatus.
As discussed above, the conventional sheet stacking apparatus can not press the sheet bundle by the substantially constant pressing force irrespective of the sheet stacking height. The conventional sheet stacking apparatus can not restrain especially the curled sheet from floating.
Yet further, the sheet processing apparatus has such a problem that processing operation time is long in terms of a layout of the pressing member and the stapler of the sheet stacking apparatus.